Note: Descriptions are shown in the official language in which they were submitted.
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PRECIPITATION HARDENING OF TANTALUM COATED METALS
BACKGROUND OF THE INVENTION
Field of the Invention
The present invention relates to a process for precipitation hardening of a
metal alloy
that has been coated with tantalum.
Introduction
Metal alloys can be protected from corrosive attack by applying a coating of
tantalum to the metal. Tantaline Inc. offers a service for vapor depositing
tantalum onto
metallic alloy substrates as a protective coating. The process requires
heating a metallic
alloy substrate in an oven to temperatures of 700-900 degrees Celsius ( C) at
which time
tantalum metal precursor is vaporized and deposited onto the substrate.
Unfortunately,
heating the substrate metal alloy to such a high temperature can partially or
completely
anneal the substrate metal alloy thereby causing the substrate metal alloy to
lose some of its
desired physical properties such as hardness, tensile modulus and compressive
modulus.
Such is the case when the metal alloy substrate is a precipitation hardened
(PH) metal alloy
where that tantalum coating process results in loss of physical properties
characteristic of
being precipitation hardened.
It is desirable to discover how to obtain a tantalum coated PH metal alloy
that
benefits from both the durable tantalum coating and the precipitation hardened
properties of
the metal substrate.
BRIEF SUMMARY OF THE INVENTION
The present invention provides a process for producing a PH metal alloy
substrate
having protective properties of tantalum coating while also having the
improved physical
properties characteristic of being a PH metal alloy such as greater hardness,
tensile modulus
and compressive modulus.
Precipitation hardening, or regenerating precipitation hardening, of a
tantalum
coated metal alloy substrate without compromising the benefits of the tantalum
coating is
not a straightforward process, as was discovered while developing the present
invention.
Precipitation hardening requires heating a metal alloy to a particular
temperature followed
by rapid cooling of the material. Heating the tantalum-coated metal alloy in
the presence of
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air at a temperature above 300 C results in the tantalum coating becoming
undesirably
oxidized and brittle due to reaction with gasses in the air such as oxygen and
nitrogen.
Heating the tantalum-coated metal alloy in a tantalum-inert gas atmosphere to
avoid making
the tantalum coating brittle prevented sufficiently rapid cooling of the metal
alloy core so as
to preclude achieving precipitation hardening.
The present invention actually serves to solve not only the general problem of
how
to induce or restore PH properties to a tantalum-coated metal alloy substrate,
but
additionally how to accomplish such a solution without causing the tantalum
coating to
become brittle and/or spall off from the metal alloy substrate.
Surprisingly, the present invention is a result of discovering that a tantalum-
coated
metal alloy substrate can be precipitation hardened without causing the
tantalum coating to
become brittle by conducting the necessary heating steps under a tantalum-
inert gas and
cooling steps under a flow of relatively cool tantalum-inert gas. Suitable
tantalum-inert gas
includes noble gasses and combinations of noble gasses. Air is also a suitable
tantalum-inert
gas at temperatures below 300 degrees Celsius ( C). The tantalum-inert gas
does not react
with the tantalum coating and does not diffuse appreciably into tantalum
coating, thereby
preventing the coating from becoming brittle. Flowing a cool tantalum-inert
gas over the
tantalum-coated metal alloy substrate allows for rapid cooling of the metal
alloy substrate
thereby allowing for precipitation hardening to occur.
In a first aspect, the present invention is a process comprising: (a)
providing a
tantalum-coated metal alloy substrate; (b) heat annealing the tantalum-coated
metal alloy
substrate by heating to an annealing temperature for the tantalum-coated metal
alloy
substrate, holding at the annealing temperature for a period of time and then
quenching to a
temperature below 50 degrees Celsius; (c) heating the tantalum-coated metal
substrate to the
precipitation hardening temperature of the metal alloy substrate; and (d)
cooling the
tantalum-coated metal alloy substrate to a temperature below 50 degrees
Celsius; wherein
the process is further characterized by carrying out steps (b)-(d) under a
tantalum-inert gas
atmosphere and by quenching in step (b) and step (d) being carried out by
flowing a
tantalum-inert gas having a temperature of less than 50 degrees Celsius over
the tantalum-
coated metal alloy substrate.
The present invention is useful for preparing tantalum-coated PH metal alloy
substrates.
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DETAILED DESCRIPTION OF THE INVENTION
The process of the present invention requires providing a tantalum-coated
metal
alloy substrate. Tantalum coating of metal substrates is a known technology
and is
commercially practiced by companies such as Tantaline, Inc., Ultramet COT and
others.
The metal alloy substrate that is tantalum-coated is a material that can
undergo precipitation
hardening. Prepare the tantalum-coated metal alloy substrate by providing a
precipitation
hardened metal alloy substrate and applying tantalum to the substrate while
sustaining a
temperature greater than 700 degrees Celsius ( C). The resulting tantalum-
coated metal
alloy substrate has a homogeneous tantalum coating over the metal alloy
substrate. Such a
high temperature coating process is advantageous over lower temperature (below
700 C)
metal coating processes, such a sputter coating, because the lower temperature
processes
produce non-homogeneous metal coating over a substrate and do not result in a
reinforcing
intermetallic layer of tantalum with the metal alloy substrate. A homogeneous
coating is
desirable for optimal substrate protection by the tantalum coating.
Precipitation hardening, also known as heat-aging, is a technique for
increasing the
compressive strength of malleable metal alloys. Examples of suitable metal
alloys that can
undergo precipitation hardening include aluminum alloys, magnesium alloys,
nickel alloys
and stainless steels. The tantalum-coated metal alloy substrate is desirably
selected from
tantalum coated magnesium alloys, nickel alloys and stainless steels. The
process of the
present invention is particularly desirable for tantalum-coated stainless
steel alloys. Specific
examples of suitable stainless steel alloys include any one or any combination
of more than
one of the following grades of stainless steel: 17-PH, 17-7PH, 13-8PH, 15-5PH.
The process of the present invention requires heat annealing the tantalum-
coated
metal alloy substrate. Heat annealing required heating the tantalum-coated
metal alloy
substrate to a high enough temperature (annealing temperature) for a long
enough period of
time to allow dissolution of the precipitant phase in the metal alloy and then
cooling
(quenching) the tantalum-coated metal alloy substrate. Desirably, heat anneal
by heating
the tantalum-coated metal alloy substrate to a temperature above the critical
temperature for
the metal alloy of the tantalum-coated metal alloy substrate and holding for a
period of
time. The critical temperature for a metal alloy is typically readily
available in metallurgy
handbooks. Annealing can also be done by heating to sub-critical temperature,
but that is a
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less desirable process for the present invention. For stainless steel alloys
it is desirable to
heat the tantalum-coated metal alloy substrate to a temperature in a range of
250 degrees or
higher, preferably 650 C or higher and yet more preferably 1000 C or higher.
At the same
time, it is common to heat to a temperature of 1500 C or lower.
Hold the tantalum-coated metal alloy substrate at the annealing temperature
for a
period of time long enough to allow the alloy solute phase to dissolve,
preferably
completely. Desirably, hold the tantalum-coated metal alloy substrate at the
annealing
temperature for 30 minutes or longer, preferably 45 minutes or longer, more
desirably one
hour or longer. While there is no known upper technical limit for how long to
hold the
material at the annealing temperature it is typical for practical purposes to
hold it at
annealing temperature for 48 hours or less, preferably 24 hours or less, still
more preferably
12 hours or less.
Quench the tantalum-coated metal alloy substrate by quickly cooling to a
temperature below 50 C, preferably to a temperature below 35 C. Rapid
quenching in the
annealing step is important to preserve very small, nanoparticle-like
precipitate domains
needed for successful precipitation hardening. However, if the tantalum-coated
metal alloy
substrate is cooled too quickly, the tantalum-coating and metal alloy
substrate can change
dimensions at sufficiently different rates so as to result in delamination of
the tantalum-
coating from the meal alloy substrate (spalling of the tantalum coating).
Therefore, it is
desirable to cool as rapidly as possible while avoiding delamination of the
tantalum coating.
Typically, it is desirable to cool the tantalum-coated metal alloy substrate
at a rate in a range
of 200 C per hour to 300 C per hour, preferably approximately 250 C per hour.
The annealing step is important in order to properly prepare the metal alloy
for
precipitation hardening. Annealing dissolves the solute phase, preferably
completely. Non-
dissolved solute in the metal alloy can form domains that are large enough to
hinder
physical property enhancement during precipitation hardening.
After quenching in the heat annealing step, precipitation harden the tantalum-
coated
metal alloy substrate. Precipitation hardening includes heating to a
precipitation hardening
temperature, holding at or above the precipitation hardening temperature for a
period of time
and then cooling.
Heat the tantalum-coated metal alloy substrate to the precipitation hardening
temperature of the metal alloy substrate. The precipitation hardening
temperature of a metal
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alloy is the temperature at which the material will produce fine particles of
impurity (solute)
phase necessary for precipitation hardening to occur. Precipitation hardening
temperatures
for materials can be found in readily available resources such as in the
procedure for ASTM
A693-13 (Precipitation-Hardening Stainless and Heat-Resisting Steel Plate,
Sheet, and
Strip). As examples, the precipitation hardening temperature is 475 C (890
degrees
Fahrenheit ( F)) or higher, typically 482 C (900 F) or higher, and often 496 C
(925 F) or
higher, 510 C (950 F) or higher, and can be 535 C (995 F), 565 C (1050 F) or
higher. At
the same time, the precipitation hardening temperature is generally 1100 C
(2012 F) or
lower, typically 1000 C (1800 F) or lower, and can be 925 C (1700 F) or lower,
900 C
(1173 F) or lower, 800 C (1073 F) or lower, 700 C (1292 F) or lower, 600 C
(1112 C) or
lower and even 550 C (1022 F) or lower.
Precipitation hardening can include multiple steps of heating and cooling of
the
substrate as described in ASTM A693-13.
Hold the tantalum-coated alloy substrate at or above its precipitation
hardening
temperature for a period of time, typically for 30 minutes or more, preferably
45 minutes or
more, more preferably an hour or more, still more preferably two hours or
more, even more
preferably three hours or more and possibly four hours or more. Generally, the
tantalum-
coated metal alloy substrate is held at the precipitation hardening
temperature for less than
hours, preferably less than ten hours and can be less than five hours.
20 The precipitation hardening temperature and time at which the tantalum-
coated
metal alloy substrate is held at the precipitation hardening temperature
determines the final
properties of the metal alloy substrate. Therefore, variations in temperature
and how long
the metal alloy substrate is held at that temperature can be varied depending
on the end
properties desired.
Cool the tantalum-coated metal alloy substrate down to a temperature below 50
C.
Desirably, cool the tantalum-coated metal substrate down to a temperature
below 50 C at an
average cooing rate of 100 C per hour or faster, preferably a rate of 125 C
per hour or faster
and more preferably at a rate of 150 C per hour or faster under a flow of
tantalum-inert gas
having a temperature of less than 50 C. It is important to cool no slower than
100 C per
hour in order to achieve a desirable increase in tensile strength properties
(for example,
ultimate tensile strength or modulus) of the metal alloy substrate. At the
same time, to
avoid spalling of the tantalum coating, it is desirable to cool at a rate of
300 C or slower,
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preferably 250 C or slower, more preferably 200 C or slower. One desirable
cooling rate is
approximately 150 C per hour.
Precipitation hardening can include multiple steps of heating and cooling as
described in ASTM A693-13, but requires at least heating step to the
precipitation
hardening temperature and then cooling as described above.
It is important in the process of the present invention that the heat
annealing and
precipitation hardening (including heating to precipitation hardening
temperature, holding at
that temperature and then cooling) be done under a tantalum-inert gas
atmosphere to
preclude embrittlement and/or spalling of the tantalum coating. Desirably, the
tantalum-
inert gas atmosphere contains 99.99 mole-percent (mol%) or higher, preferably
99.995
mol% or higher, still more preferably 99.999 mol% or higher, yet more
preferably 99.9995
mol% or higher and even more preferably 99.9999 mol% or higher of a tantalum-
inert gas
based on total moles of gas in the tantalum-inert gas atmosphere.
A tantalum-inert gas is a gas that does not react with tantalum, and
preferably will
not diffuse into tantalum, at temperatures in a range of 200-2000 C. For
avoidance of
doubt, the tantalum-inert gas is a gas in the temperature range of use.
Examples of
tantalum-inert gases include gases selected from the noble gases (helium,
neon, argon,
krypton, and xenon) including any combination of more than one noble gas. Air,
and any
component of air, is also a suitable tantalum-inert gas at temperatures up to
300 C.
The steps that are conducted under a tantalum-inert gas atmosphere cannot
satisfactorily be conducted under a gas atmosphere that contains appreciable
amounts of gas
that is reactive with tantalum. If the annealing and precipitation hardening
steps are
conducted under a gas that is reactive with tantalum then the tantalum coating
undergoes a
chemical reaction and becomes undesirably brittle. For example, oxygen,
nitrogen and
hydrogen are all reactive with tantalum at temperatures above 300 C, causing
it to become
brittle. Carbon dioxide, ammonia and hydrocarbons are also known to react with
tantalum
at temperatures in the 300-2000 C range. Desirably, the tantalum inert gas
atmosphere
contains less than 0.01 mole-percent (mol%), preferably 0.005 mol% or less,
more
preferably 0.001 mol% or less, still more preferably 0.0005 mol% or less, even
more
preferably 0.0001 mol% or less of any combination of oxygen, nitrogen and
hydrogen (and
more preferably any combination of oxygen, nitrogen, hydrogen, carbon dioxide,
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hydrocarbons and ammonia) based on total moles of gas molecules in the
tantalum-inert gas
atmosphere.
Moreover, the cooling (quenching) step after heating to the precipitation
hardening
temperature (and the quenching step during annealing) must be done by flowing
a tantalum-
inert gas having a temperature of less than 50 C over the tantalum-coated
metal substrate. It
is important to flow the tantalum-inert gas over the tantalum-coated substrate
during cooling
in order to cool the tantalum-coated substrates at a sufficient rate. Flowing
relatively cool
(less than 50 C) tantalum inert gas over the tantalum-coated substrate
efficiently removes
heat from the tantalum-coated substrate thereby cooling the substrate at a
satisfactory rate.
The relatively cool gas must be a tantalum-inert gas to preclude damage, such
as
embrittlement and/or spalling, of the tantalum coating.
Examples
Prepare samples using stainless steel tensile bars. The stainless steel is SS
17-4PH
or 15-5PH condition 900 (H900) stainless steel as indicated below. The tensile
bars are
ASTM E8 Standard Subsize Tensile Bars with a rectangular cross section.
Evaluate the samples by measuring tensile properties and corrosion resistance.
Characterize tensile properties according to ASTM E8-09. Determine corrosion
resistance
by submerging the sample in 20-35 wt% hydrochloric acid solution in water at
75 C for 48
hours and evaluating samples for any signs of pitting.
Comparative Example A. For Comparative Example A, evaluate a 17-4PH stainless
steel tensile bar without any further treatment (that is, without a tantalum
coating or any
thermal conditioning). Comparative Example A has an ultimate tensile strength
(UTS) of
1448 MegaPascals (MPa). Comparative Example A nearly dissolve in the corrosion
test
and could not be recovered.
Comparative Example B. Comparative Example B is the same as Comparative
Example A except the tensile bar is coated with a tantalum coating by
Tantaline Inc.
according to their commercial coating technology. Characterize Comparative
Example B
after coating with tantalum. Comparative Example B has an average UTS of 1065
MPa
(average of two measurements: 1027 MPa and 1103 MPa). No signs of pitting were
observed in the corrosion test.
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Example /. Example 1 is the same as Comparative Example B except further
subjected to an annealing and precipitation hardening thermal reconditioning
profile after
coating with tantalum and prior to characterizing corrosion resistance and
tensile properties.
Use a thermal reconditioning profile as shown in Table 1 where precipitation
hardening is
done directly after annealing. Subject the sample to the thermal
reconditioning profile in an
argon atmosphere by flowing argon (at least 99.999 mole-percent argon) over
the sample.
The argon is at ambient temperature (approximately 23 C).
Table 1
Elapsed Time Temperature Rate of Temperature Change
(minutes) ( C) ( C/hour)
Annealing
0 32 Not Applicable (start temp)
69 1038 875
129 1038 0
369 32 -252
Precipitation Hardening
399 32 0
468 485 394
528 485 0
708 32 -151
Example 1 has an average UTS of 1280 MPa (average of three measurements: 1289
MPa, 1282 MPa and 1269 MPa). No pitting was observed in the corrosion test.
The tensile properties and corrosion properties of Comparative Examples A and
B
and Example 1 illustrate that the process of the present invention provides a
metal alloy
substrate that benefits from the corrosion resistance of a tantalum coating
and greater tensile
strength not normally present in a tantalum coated sample. A comparison of the
tensile
strength of Comparative Examples A and B reveals how the tantalum coating
process
reduces the tensile strength of the metal alloy substrate. Example 1
illustrates that the
process of the present invention restores at least a portion of the tensile
strength of the metal
alloy substrate while retaining the corrosion resistance of the tantalum
coating.
Example 2. Prepare Example 2 in a similar manner as Example 1, except use a 15-
5PH stainless steel tensile bar coated with a tantalum coating by Tantaline
Inc. Further
subject the tantalum coated bar to an annealing and precipitation hardening
thermal
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reconditioning profile after being tantalum coated and prior to characterizing
corrosion
resistance and tensile properties. Use a thermal reconditioning profile as
shown in Table 1
where precipitation hardening is done directly after annealing. Subject the
resulting sample
to thermal reconditioning profile in an argon atmosphere by flowing argon
(99.999 mole-
percent argon) at a temperature of approximately 23 C over the sample. Example
2 has an
average UTS of 1344 MPa. No pitting was observed in the corrosion test.
Example 2 further illustrates the benefit of the process of the preset
invention using a
stainless steel substrate different from Example 1.
When the annealing and precipitation hardening of the tantalum-coated metal
alloy
substrate of Example 1 and Example 2 were done in an air atmosphere, the
tantalum coating
oxidized and failed thereby reducing the corrosion resistance of the resulting
samples.
Similar results are expected if the annealing and precipitation hardening are
done in any
other non-tantalum-inert gas atmosphere.
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